1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a phase change memory device and a method of forming the same.
2. Description of the Related Art
A phase change random access memory (PRAM) is a memory device that uses a material that changes its phase, e.g., amorphous to crystalline, as a mechanism to store data. Generally, PRAM memories may resemble dynamic random access memory (DRAM) devices with their capacitors replaced by an appropriate phase change material. Note that phase change materials have different resistance values as a function of their underlying phase state, which can vary from a generally amorphous state to a more crystalline state. The amorphous state of a phase change material will typically have a higher specific resistance than that of the crystal state, and using this difference of specific resistance it is possible to determine whether data recorded on the phase change material is a logic “1” or a logic “0” by detecting a voltage change and/or a current change caused by difference of this specific resistance.
A representative example of a phase change material may include a compound containing germanium (Ge), tellurium (Te) and antimony (Sb)—generally referred to as GST. A PRAM using this phase change material can have the benefit of being non-volatile, i.e., it can maintain its stored data even when power is removed. Another benefit of PRAM memory is its high durability in that data can be written and rewritten billions of times.
The phase of a phase change material memory cell can be changed to an amorphous state or a crystalline state based on an amount of heat applied thereto. More particularly, the phase change material may be converted into an amorphous state by supplying heat of temperature higher than a melting point to the phase change material during a first time interval. In contrast, the phase change material may be converted into a crystalline state by supplying heat lower than the melting point to the phase change material during a second time interval that is longer than the first time interval.
Generally, PRAM devices use Joule (ohmic) heating to set the state of its phase change materials. That is, by forming a contact plug-shaped heater on the lower surface of the phase change material and applying a program current through the heater the temperature of the heater (along with the phase change material) may be controlled so that the phase change material may be converted into an amorphous state or a crystalline state. Note that the heater may be connected to source/drain regions of a metal oxide semiconductor (MOS) transistor (formed on a semiconductor substrate) acting as the current switch for the heater.
It should be appreciated that the heat required for converting a state of the phase change material is generally very high. For example, the GST material discussed above requires high temperature of at least about 600° C. Generating such a high temperature can greatly add to the power consumption of the PRAM. Also, in situations where the channel width of the MOS transistor is widened in order to supply high program currents, the overall area of the PRAM cell proportionately increases, which tends to make high integration more difficult. Thus, new technology relating to PRAM devices is desirable.